The present invention relates to a method for driving a fluorescent lamp, in one embodiment for igniting the fluorescent lamp, and a lamp ballast.
Lamp ballasts for fluorescent lamps or gas discharge lamps usually include a half-bridge circuit and a series resonant circuit connected to the half-bridge circuit, which series resonant circuit can be connected to the fluorescent lamp. In this case, the half-bridge circuit serves for exciting the series resonant circuit and for this purpose generates an AC voltage from a DC voltage present across the half-bridge.
A start phase of a lamp ballast includes, for example, a preheating phase and an ignition phase for igniting the lamp. During the preheating phase, incandescent filaments of the lamp are heated by setting a frequency of the AC voltage, which is referred to hereinafter as excitation frequency, in such a way that it lies above the resonant frequency of the series resonant circuit. During the ignition phase, the excitation frequency is increasingly reduced in the direction of the resonant frequency of the resonant circuit, with the aim of increasing a voltage across the fluorescent lamp by using a resonance magnification to an extent such that an ignition voltage of the lamp is obtained and the lamp ignites. During an operating phase after ignition of the lamp, the excitation frequency can then be reduced still further.
During the ignition phase, it should in this case be ensured, on the one hand, that the voltage across the lamp can rise up to the value of the ignition voltage. On the other hand, it should be ensured for safety reasons that the voltage does not rise to an arbitrary extent, for example, when the lamp does not ignite on account of a defect or when no lamp is connected to the resonant circuit. In this respect, U.S. Pat. No. 6,525,492 proposes detecting a current through the half-bridge and immediately switching off the half-bridge if the current exceeds a predetermined threshold value.
For cost reasons, the coil of the resonant circuit is often dimensioned such that it already operates in the vicinity of its magnetic saturation if the lamp voltage lies in the region of the ignition voltage. As is known, the effective inductance of a coil decreases upon transition to the saturation range. If, during the ignition process, an excitation frequency is attained at which the coil begins to go to saturation, then the resonant frequency of the series resonant circuit increases owing to the decreasing inductance of the coil, and a separation between the instantaneous excitation frequency and the resonant frequency decreases. With the excitation frequency remaining constant, the voltage rises further as a result, the coil goes further to saturation and the resonant frequency further approaches the instantaneous excitation frequency. As a result of this positive feedback effect explained, instabilities can arise in the setting of the ignition voltage.
For these and other reasons, there is a need for the present invention.